Abstract
Blade tip cavitation is a well-known phenomenon that affects the performance of large-diameter Kaplan turbines and induces structural vibration. Injection of pressurized air has been found to yield promising results in reducing those damaging effects. In this work, the results of an experimental test of air injection on a 9.5-m-diameter Kaplan turbine are reported. Experiments were performed for several load conditions and for two different net heads. Accelerations, pressure pulsation and noise emission were monitored for every tested condition.Results show that, at the expense of a maximum efficiency drop of 0.2%, air injection induces a decrease on the level of vibration from 57% up to 84%, depending on the load condition. Such decrease is seen to be proportional to the air flow rate, in the range from 0.06 to 0.8‰ (respect to the discharge at the best efficiency point).
Highlights
Large-diameter Kaplan turbines may undergo vibration and erosion at the discharge ring due to the development of tip vortex cavitation phenomena
At the expense of a maximum efficiency drop of 0.2%, air injection induces a decrease on the level of vibration from 57% up to 84%, depending on the load condition
In agreement with the results found on a physical scale model, air injection proved to be an effective method for the mitigation of tip blade cavitation in a Kaplan prototype turbine
Summary
Large-diameter Kaplan turbines may undergo vibration and erosion at the discharge ring due to the development of tip vortex cavitation phenomena. Severe erosion problems can compromise the lifetime of the machine or cause expensive repair stops. As such issues are difficult to foresee at the design stage, even with the help of CFD simulations and model tests, mitigation strategies have become an important subject. It was found recently [2] that miniscule amounts of non-condensable gas (air) into the shear layer of a partial cavity on a wedge could reduce the void fraction (i.e., the ratio of gas to water volumes), reducing the potential energy available for noise, vibration and erosion. Air injection has shown potential in preventing erosion due to cavitation in such cases as the chute of spillways of hydroelectric dams [3].
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